1. Field of the Invention
The present invention relates to light valves, and in particular, to light valves utilizing self-aligned thin liquid crystal pixel cells having metal electrodes that are electronically isolated from one another by dielectric spacer walls flush with the electrode surface.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These display devices typically include a light source which passes light through a light valve.
One of the methods for producing colors in a liquid crystal display is to sequentially project light having a wavelength corresponding to a primary color onto a single light valve. Color sequential light valves create a spectrum of color within the range of the human perception by switching between a set of discrete primary colors. Typically, red, green, and blue are the primary tri-stimulus colors used to create the remaining colors of the spectrum.
Specifically, during projection of each primary color, the light intensity is modulated such that combination of the intensities of the primary colors in sequence produces the desired color. The frequency of switching between the primary wavelengths by the light valve should be sufficiently rapid to render discrete primary states indistinguishable to the human eye. Two factors dictate the minimum frequency necessary for switching.
The first factor is the ability of the human eye to detect the discrete primary colors (e.g., red, green, blue). At slower than ideal switching speeds, the human eye will detect a flicker and the primaries may not blend.
The second factor determining the frequency of switching is the video refresh rate. During display of video images, the individual frames must be refreshed at frequencies undetectable to the human eye.
The net frequency of switching demanded by the combination of sequential color blending and video refreshing is beyond the capabilities of light valves that utilize thick (&gt;1 .mu.m) liquid crystal (LC) transducers. However, thin (&lt;1 .mu.m) liquid crystal transducers have been successfully fabricated. These thin LC transducers demonstrate adequate color sequential blending at video refresh rates. One example of such a thin LC transducer pixel cell structure is disclosed in U.S. Pat. No. 5,706,067, to Colgan et al.
In general, the conventional thin LC transducer pixel cells possess enhanced responsiveness due to the decreased volume of liquid crystal material between the top and bottom plates. A smaller volume enables the liquid crystal to shift orientation more quickly and in response to a lower applied voltage.
FIG. 1A shows a top view of a conventional thin LC transducer pixel cell. FIG. 1B shows a cross-sectional view of the thin liquid crystal transducer along line A-A' of FIG. 1A.
Thin LC transducer pixel cell 100 comprises a layer of liquid crystal (LC) material 102 sandwiched between a top plate 104 and a bottom plate 106. Top plate 104 is a translucent material, typically glass. Bottom plate 106 is a reflective pixel electrode layer.
Pixel electrode layer 106 is delineated into individual pixel electrodes 130 by intervening trenches 118. Pixel electrode layer 106 lies on top of an upper intermetal dielectric layer 112 that is one component of an interconnect scheme. The interconnect overlies a capacitor structure formed within an underlying silicon substrate (not shown). Upper intermetal dielectric layer 112 electrically insulates pixel electrode 130 from lower metallization layer 114. The underlying capacitor structure is in electrical communication with pixel electrode 130 through metal-filled via 116.
FIGS. 2AA-2DB illustrate the conventional process for forming a thin LC transducer pixel cell. For purposes of convention, all FIG. 2.sub.-- A illustrate a top view of the pixel cell, all FIG. 2.sub.-- B illustrate a cross-sectional view of the pixel cell along line A-A' of the FIG. 2.sub.-- A.
FIGS. 2AA-2AB illustrate the starting point for the conventional process for fabricating a thin LC transducer pixel cell. Starting structure 200 is created by forming an upper intermetal dielectric layer 212 over a lower interconnect metallization layer 214. A central portion of upper intermetal dielectric layer 212 is then etched to form via 216. A liner (not shown) typically composed of a Ti/TiN layer combination, is then formed on the walls of via 216, and via 216 is filled with metal (typically CVD Tungsten). Excess metal is then removed from the surface of upper dielectric layer 212, typically by a combination of etching and chemical-mechanical polishing (CMP).
FIGS. 2BA-2BB illustrate formation of the metal pixel electrode in accordance with the conventional process. Metal pixel electrode layer 206 is formed over the entire surface of the pixel cell.
FIGS. 2CA-2CB illustrate patterning of a photoresist mask 207 over pixel electrode layer 206. FIGS. 2DA-2DB show the etching of regions of pixel electrode layer 206 unmasked by photoresist 207, to form a plurality of intersecting trenches 218, followed by stripping of photoresist mask 207. Intersecting trenches 218 in turn define a plurality of discrete pixel cell electrodes 230.
Fabrication of the thin LC transducer pixel cell is completed by forming an alignment surface (not shown) for the LC material positioned on top of the pixel electrode. Forming this alignment surface is a two step process. First, a dielectric film (typically polyimide) is deposited on top of the pixel electrode. Second, the dielectric film is scored by a rubbing wheel, which traverses the surface of the pixel cell and gouges the alignment surface in a uniform direction. Liquid crystal material is then placed within the cell, and a top glass plate is secured to the tops of the support pillars.
The conventional fabrication process described above in FIGS. 2AB-2DB is adequate to produce functional thin LC transducer pixel cells. However, the conventional process flow suffers from a number of serious disadvantages.
One problem with the process described above is that it creates significant pixel surface topology that can result in optical degradation.
Liquid crystal material overlying the pixel electrode has the propensity to align and/or tilt with the grooves caused by any topology present on the surface of the pixel cell. LC alignment is a critical system attribute. The alignment of the twisted nematic LC dictates which polarization of incident light will pass through the LC's volume. In the context of a complete system, this alignment of the LC material defines either the black or white extreme of the light valve's gray scale. As a result, non-uniformity in alignment due to the presence of surface topology will translate into a poorly constructed display.
In FIGS. 1A-1B, the pixel array includes a plurality of discrete pixel electrodes that are electronically isolated from one another by a series of intersecting trenches having side walls and a trench bottom. These surface topology features can interact with the overlying LC material, causing it to misalign. This misalignment can cause unwanted distortion of the image formed using the light valve.
In addition, surface topology of the pixel cell can also interact directly with incident light, causing reflection that is not harmonious with that of the main body of the pixel electrode. The interaction of light with the pixel topography is due to the isolation edges of the pixel. For example, in the pixel array shown in FIGS. 1A-1B light will scatter from the sidewalls and bottom of the trenches present at the pixel edges. This unwanted scattering reduces the specular reflection of the pixel, and increases optical cross-talk between pixels.
Therefore, there is a need in the art for a process of forming a thin LC transducer pixel cell that creates a minimum of surface topology on the pixel cell surface while maintaining electrical isolation between pixel electrodes.
A second problem associated with the conventional method of fabricating light valves is etching of the metal pixel electrode layer to form discrete pixel electrodes. This etch step degrades the reflectance of the pixel electrode. When the metal pixel electrode layer is freshly deposited, it is extremely smooth and exhibits high reflectance desirable for optimum performance. However, etching the metal pixel electrode layer to form the trenches isolating individual pixel electrodes can roughen and/or oxidize the surface of the pixel electrode layer, lowering its reflectance.
Therefore, there is a need in the art for a process of forming a thin LC transducer pixel cell that avoids etching or otherwise roughening the surface of the pixel electrode.